Door bell

A doorbell is a signaling device typically placed near a door to a building's entrance. When a visitor presses a button the bell rings inside the building, alerting the occupant to the presence of the visitor. Although the first doorbells were mechanical, activated by pulling a cord, modern doorbells are generally electric switch.

William Murdoch, a Scottish inventor, installed a number of his own innovations in his house, built in Birmingham in 1817; one of these was a loud doorbell, that worked using a piped system of compressed air. A precursor to the electric doorbell, specifically a bell that could be rung at a distance via an electric wire, was invented by Joseph Henry around 1831. By the early 1900s, electric doorbells had become commonplace.

In most wired systems, a button on the outside next to the door, located around the height of the doorknob, activates a signaling device (usually a chime, bell, or buzzer) inside the building. Pressing the doorbell button, a single-pole, single-throw (SPST) pushbutton switch momentarily closes the doorbell circuit. One terminal of this button is wired to a terminal on a transformer. A doorbell transformer steps down the 120 or 240-volt AC electrical power to a lower voltage, typically 10 to 20 volts. The transformer's other terminal connects to one of three terminals on the signaling device. Another terminal is connected to a wire that travels to the other terminal on the button. Some signaling devices have a third terminal, which produces a different sound. If there is another doorbell button (typically near a back door), it is connected between the transformer and the third terminal. The transformer primary winding, being energized continuously, does consume a small amount (about 1 to 2 W) of standby power constantly; systems with lighted pushbutton switches may consume a similar amount of power per switch. The tradeoff is that the wiring to the button carries only safe, low voltage isolated from earth ground.

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